Halo 1 and 2 are already on PC, they just need a proper steam release. Pretty sure they will make it there eventually. Can't say the same about the rest of them though.
It's a shame Games for Windows was so crap. I don't see MS ever putting their biggest and best games on Steam but I could so them having Xbox/PC releases on their own store system. Be cool to get Halo 5 on PC :P
You must mean Games for Windows Live, not Games for Windows. GFW is fine. GFWL is one steaming pile. Because of it, I can't get Bulletstorm to work now. Good thing it's the only GFWL game I have.